Xylenes are found in fractions from coal tar distillate, petroleum reformates and pyrolysis liquids in admixture with other compounds of like boiling point. The aromatic components are readily separated from non-aromatics by solvent extraction. Distillation provides a fraction consisting essentially of C.sub.8 aromatics. As will appear below, o-xylene is separable from other C.sub.8 aromatics by fractional distillation, and p-xylene is separable by fractional crystallization. Present demand is largely for p-xylene and it has become desirable to convert m-xylene, the principal xylene present in the feed stream, to the more desired p-xylene.
Since the announcement of the first commercial installation of Octafining in Japan in June, 1958, this process has been widely installed for the supply of p-xylene. See "Advances in Petroleum Chemistry and Refining" volume 4, page 433 (Interscience Publishers, New York 1961). That demand for p-xylene has increased at remarkable rates, particularly because of the demand for terephthalic acid to be used in the manufacture of polyesters.
Typically, p-xylene is derived from mixtures of C.sub.8 aromatics separated from such raw materials as petroleum naphthas, particularly reformates, usually by selective solvent extraction. The C.sub.8 aromatics in such mixtures and their properties are:
______________________________________ Density Freezing Boiling Lbs./U.S. Point .degree.F. Point .degree.F. Gal. ______________________________________ Ethyl benzene -139.0 277.1 7.26 P-xylene 55.9 281.0 7.21 M-xylene -54.2 282.4 7.23 O-xylene -13.3 292.0 7.37 ______________________________________
Principal sources at present are catalytically reformed naphthas and pyrolysis distillates. The C.sub.8 aromatic fractions from these sources vary quite widely in composition but will usually be in the range 10 to 32 wt. percent ethyl benzene with the balance, xylenes, being divided approximately 50 wt. percent meta, and 25 wt. percent each of para and ortho.
In turn, calculated thermodynamic equilibra for the C.sub.8 aromatic isomers at Octafining conditions are:
______________________________________ Temperature 850.degree. F. ______________________________________ Wt. % Ethyl benzene 8.5 Wt. % para xylene 22.0 Wt. % meta xylene 48.0 Wt. % ortho xylene 21.5 TOTAL 100.0 ______________________________________
An increase in temperature of 50.degree. F. will increase the equilibrium concentration of ethyl benzene by about 1 wt. percent, ortho-xylene is not changed and para and meta xylenes are both decreased by about 0.5 wt. percent.
Individual isomer products may be separated from the naturally occurring mixtures by appropriate physical methods. Ethyl benzene may be separated by fractional distillation although this is a costly operation. Ortho xylene may be separated by fractional distillation and is so produced commercially. Para xylene is separated from the mixed isomers by fractional crystallization.
As commercial use of para and ortho xylene has increased there has been interest in isomerizing the other C.sub.8 aromatics toward an equilibrium mix and thus increasing yield of the desired xylenes.
Octafining process operates in conjunction with the product xylene or xylenes separation processes. A virgin C.sub.8 aromatics mixture is fed to such a processing combination in which the residual isomers emerging from the product separation steps are then charged to the isomerizer unit and the effluent isomerizate C.sub.8 aromatics are recycled to the product separation steps. The composition of isomerizer feed is then a function of the virgin C.sub.8 aromatic feed, the product separation unit performance, and the isomerizer performance.
The isomerizer unit itself is most simply described as a single reactor catalytic reformer. As in reforming, the catalyst contains a small amount of platinum and the reaction is carried out in a hydrogen atmosphere.
Octfiner unit designs recommended by licensors of Octafining usually lie within these specification ranges:
______________________________________ Process Conditions ______________________________________ Reactor Pressure 175 to 225 PSIG Reactor Inlet Temperature Range 830-900.degree. F. Heat of Reaction Nil Liquid Hourly Space Velocity 0.6 to 1.6 Vol/Vol/Hr. Number of Reactors, Downflow 1 Catalyst Bed Depth, Feet 11 to 15 Catalyst Density, Lb/Cu. Ft. 38 Recycle Circulation, Mols Hydrogen/Mol Hydrocarbon Feed 7.0 to 14.0 Maximum Catalyst Pressure Drop, PSI 20 ______________________________________
It will be apparent that under recommended design conditions, a considerable volume of hydrogen is introduced with the C.sub.8 aromatics. In order to increase throughput, there is great incentive to reduce hydrogen circulation with consequent increase in aging rate of the catalyst. Aging of the catalyst occurs through deposition of carbonaceous materials on the catalyst with need to regenerate by burning off the coke when the activity of the catalyst has decreased to an undesirable level. Typically the recommended design operation will be started up at about 850.degree. F. with reaction temperature being increased as needed to maintain desired level of isomerization until reaction temperature reaches about 900.degree. F. At that point the isomerizer is taken off stream and regenerated by burning of the coke deposit.
Because of its capability to convert ethyl benzene, Octafining can accept a charge stream which contains that component. Normally, a portion of the ethyl benzene is removed by fractional distillation before the charge is processed. If no attempt is made to reduce ethyl benzene below a few percent by weight, this can be accomplished inexpensively and the ethyl benzene recovered is in usable form as a relatively pure chemical, e.g., for dehydrogenation to styrene.
The Octafiner is in a "loop" which includes means for separation of desired xylenes; p-xylene by crystallization and, possible, o-xylene by distillation. The C.sub.8 stream stripped of desired xylenes returns to the Octafiner where more of the desired xylenes are generated, for example by isomerization of m-xylene. It will be apparent that ethyl benzene will tend to build up in the loop as other components are removed. The Octafining catalyst has capability for converting ethyl benzene, thus counteracting that tendency. It, the Octafining catalyst, has the disadvantage that it is a hydrocracking catalyst due to the acid function of its silica/alumina base and its content of hydrogenation/dehydrogenation metal of the platinum group. In addition to converting ethyl benzene, this catalyst also causes net loss of xylenes.
Other catalysts have recently been identified as behaving in the same fashion as Octafining catalyst for isomerization of xylenes in C.sub.8 aromatic fractions accompanied by conversion of ethyl benzene. These new catalysts include zeolites of the ZSM-5 type, zeolite ZSM-12 .[.and zeolite ZSM-21.]. .Iadd., zeolite ZSM-35 and zeolite ZSM-38.Iaddend.. ZSM-5 type includes zeolite ZSM-5 as described in Argauer and Landolt Pat. No. 3,702,886, dated Nov. 14, 1972 and zeolite ZSM-11 as described in Chu Pat. No. 3,709,979, dated Jan. 7, 1973 and variants thereon. Zeolite ZSM-12 is described in German Offenlegungsschrift No. 2,213,109. The activity of these catalysts for the stated purpose and of .[.ZSM-21 is.]. .Iadd.zeolites ZSM-35 and ZSM-38 (there identified as ZSM-21) are .Iaddend.described and claimed in copending application of R. A. Morrison, Ser. No. 397,039, filed Sept. 13, 1973, the disclosure of which is hereby incorporated by reference.
In general, Octafining catalyst and the zeolite catalysts referred to above behave in about the same manner, except for their aging characteristics; decline of activity with time on stream.
A typical charge to the isomerizing reactor (effluent of crystallizer for separation of p-xylene) may contain 17 wt. percent ethyl benzene, 65 wt. percent m-xylene, 11 wt.% p-xylene and 7 wt. percent o-xylene. The thermodynamic equilibrium varies slightly with temperature in a system in which o-xylene is separated in the loop by fractional distillation prior to the crystallizer. The objective in the isomerization reactor is to bring the charge as near to theoretical equilibrium concentrations as may be feasible consistent with reaction times which do not give extensive cracking and disproportionation.
Ethyl benzene reacts through ethyl cyclohexane to dimethyl cyclohexanes which in turn equilibrate to xylenes. Competing reactions are disproportionation of ethyl benzene to benzene and diethyl benzene, hydrocracking of ethyl benzene to ethylene and benzene and hydrocracking of the alkyl cyclohexanes.
The rate of ethyl benzene approach to equilibrium concentration in a C.sub.8 aromatic mixture is related to effective contact time. Hydrogen partial pressure has a very significant effect on ethyl benzene approach to equilibrium. Temperature change within the range of Octafining conditions (830.degree. to 900.degree. F.), has but a very small effect on ethyl benzene approach to equilibrium.
Concurrent loss of ethyl benzene to other molecular weight products relates to percent approach to equilibrium. Products formed from ethyl benzene include C.sub.6 + naphthenes, benzene from cracking, benzene and C.sub.10 aromatics from disproportionation, and total loss to other than C.sub.8 molecular weight. C.sub.5 and lighter hydrocarbon by-products are also formed.
The three xylenes isomerize much more selectively than does ethyl benzene, but they do exhibit different rates of isomerization and hence, with different feed composition situations the rates of approach to equilibrium vary considerably.
Loss of xylenes to other molecules weight products varies with contact time. By-products include naphthenes, toluene, C.sub.9 aromatics and C.sub.5 and lighter hydrocracking products.
Ethyl benzene has been found responsible for a relatively rapid decline in catalyst activity of Octafining catalyst and this effect is proportional to its concentration in a C.sub.8 aromatic feed mixture. It has been possible then to relate catalyst stability (or loss in activity) to feed composition (ethyl benzene content and hydrogen recycle ratio) so that for any C.sub.8 aromatic feed, desired xylene products can be made with a selected suitably long catalyst use cycle.
A more recent development than Octafining is Low Temperature Isomerization (LTI) as described in Wise Pat. No. 3,377,400, dated Apr. 9, 1968. That process is particularly effective when the zeolite catalyst employed is ZSM-4 as described in Bowes, et al. U.S. Pat. No. 3,578,723, dated May 11, 1971.
The advantages of LTI rest in its capbility to isomerize xylenes in liquid phase at relatively low temperatures and the lack of necessity for hydrogen pressure in the reactor. The zeolite catalyst, particularly ZSM-4, ages very slowly even with hydrogen or a hydrogenation/dehydrogenation metal component of the catalyst. However, LTI has one disadvantage. It leaves ethyl benzene unchanged.
Because the ethyl benzene content of C.sub.8 aromatic fractions is unchanged in LTI operations as practiced heretofore, such operations incur severe costs in capital investment and in operating expense to dispose of the ethyl benzene in order that it shall not build up in the system. Because of the minor difference in boiling point between ethyl benzene and certain of the xylenes, complete removal of ethyl benzene from the charge is prohibitive in cost. The practical way to handle this component is to provide an additional distillation column in the loop to remove ethyl benzene at substantial cost.